1. Field of the Invention
The present invention generally relates to image forming methods and image forming apparatuses.
2. Description of the Related Art
Various methods of electrophotography are known. Generally, the surface of an image carrier is charged and then exposed to light to form an electrostatic latent image thereon. The electrostatic latent image is then developed using a toner, whereby a toner image is formed on the image carrier. The toner image on the image carrier is further transferred to a recording material as a transfer material, either directly or via an intermediate transfer member. The thus transferred toner image is fused by the application of heat and/or pressure, whereby a recorded item with the image formed thereon is obtained. The toner that may remain on the image carrier after the toner image is transferred is cleaned in a manner known in the art, such as with the use of a blade, a brush, or a roller, for example.
One direction in which the state-of-the-art electrophotographic technology is headed is the increase in the speed of color image output. In order to accelerate the output of color images, a tandem-type electrophotography system is used in which plural image carriers and developing devices are employed. In this technology, a color toner image formed on each of the image carriers is transferred to a recording material either directly or via an intermediate transfer member. Particularly, a transfer system involving an intermediate transfer member simplifies the transport of the recording material, and is also capable of handling various types of recording material. Thus, such a transfer system is adopted in various image forming apparatuses.
There is also the trend of increasing the process speed in order to achieve a further increase in speed. This has resulted in greater stress to which the toner is exposed within the system. The increase in stress to the toner leads to various problems, such as variations in the toner's charge characteristics and a decrease in toner fluidity due to the detachment of an additive from the toner surface into the toner. As a result, the rate of degradation of the toner increases. One of the measures to control such toner degradation is to increase the granule size of the additive so that the additive does not become buried in the toner. However, as the granule size of the additive increases, the additive can separate from the toner with increasing ease, often resulting in damaging the image carrier. Thus, it is difficult to prevent toner degradation.
Degradation of the toner causes changes in the amount of charged toner, charge amount distribution, or toner flow property, resulting in an increased frequency of the case of abnormal images with problems such as scumming and an image density decrease. Thus, the development of at least these abnormal images must be prevented in order to properly operate an image forming apparatus.
Japanese Laid-Open Patent Application No. 09-34243 discloses that, based on the assumption that the toner degradation is inevitable, the degraded toner is forcibly developed at certain intervals of image output, so that the toner can be collected in a cleaning step. However, such compulsory consumption of toner at regular intervals of image output produces a large amount of waste toner.
In order to reduce the amount of waste toner, the state of degradation of toner is detected so that the degraded toner can be processed efficiently based on the detected state of toner degradation. Various methods for detecting the state of degradation of toner have been proposed.
For example, Japanese Laid-Open Patent Application No. 8-123263 discloses that the potential of a developed toner layer is detected in order to determine the degree of toner degradation based on a shift in the potential. In other examples, the image density of a developed toner layer is detected, or the degradation of toner or developing agent is determined based on a combination of image density detection and toner density detection.
Japanese Laid-Open Patent Application No. 2005-62858 discloses a method for visualizing the attachment of toner to a non-image portion (fogging), which is caused by the increase of the toner with the opposite charge polarity to the normal charge polarity due to the degradation of the developing agent. In this method, the developing bias and/or the transfer bias is controlled in order to cause a stripe pattern to appear on a recording medium or an intermediate transfer member, thus enabling the detection of a fog.
Japanese Laid-Open Patent Application No. 2004-239978 discloses that the image density of a developed toner layer is detected, and a developing capacity varying unit is controlled based on a detected result, so that the image quality degradation can be controlled.
Japanese Laid-Open Patent Application No. 2007-206496 discloses that a halftone patch is produced and its density is detected. Based on a relationship between input data and the output density, the degraded toner is forcibly discharged and new toner is supplied, thus controlling the toner density and stabilizing the charging efficiency of the developing agent.
Japanese Laid-Open Patent Application No. 2004-170660 discloses that the degree of degradation of developing agent is accurately determined based on measurement values obtained by an optical toner density sensor and a permeability sensor. The image forming conditions are changed, or the developing agent in the developing unit is exchanged based on the identified degree of toner degradation, in order to form an image stably.
The foregoing examples involve detection based on the developed toner layer, wherein changes in development characteristics are detected. Development characteristics are influenced by various factors. Those factors include the charge property of the material of a carrier or the like for imparting charge to the toner, and the charge property or the powder characteristics, such as flow property, of the toner. Particularly, the influence of a change in the charged amount of toner is large, while the development characteristics are less sensitive to a change in powder characteristics of the toner such as its flow property.
Meanwhile, various methods for detecting toner degradation by detecting the image density after the transfer step have been proposed. Japanese Laid-Open Patent Application No. 2003-207958 discloses that the amount of remaining toner on the photosensitive drum is detected after a reference toner image is transferred to a transfer medium under certain transfer conditions or under transfer conditions opposite to normal conditions. Then, the amount of the oppositely charged component of the toner in the developing unit is determined. Based on the thus determined amount, the developing conditions are changed or the developing agent is replaced.
However, because the developed amount of toner or the transfer rate varies depending on the amount of charge on the toner or its powder characteristics, it cannot be determined whether the charge amount has changed due to the environment, or whether the charge amount or the powder characteristics have changed due to degradation of toner, even if the residual toner amount is detected under certain developing and transfer conditions.
Japanese Laid-Open Patent Application No. 7-92753 discloses that the developed amount of toner and the transferred amount of toner, or the developed amount of toner and the residual toner amount after the transfer step are detected by a single image density sensor in order to determine the transfer rate. When the toner characteristics are changed by degradation, the transfer conditions are adjusted so that the transfer rate remains within an appropriate range.
In this method, when detecting the developed amount of toner and the transferred amount of toner, a both-side print mode is selected and a developed image is once transferred to the recording material, and then the image is developed again and the developed amount of toner is determined, followed by detecting the transferred amount of toner on the printed surface of the inverted recording material. Thus, this method cannot be applied when an intermediate transfer member is involved.
Furthermore, when detecting the developed amount and the residual toner amount, it is necessary to detect the developed amount before the transfer step, and then develop the image and detect the residual toner amount after the transfer step. Thus, the toner that is used for the detection of the developed amount is disposed of, resulting in an increase in the waste toner amount.
Further, when detecting the developed amount, the amount of attached toner is detected after passing through the transfer area. Thus, when an intermediate transfer member is used, the developed layer comes into contact with the intermediate transfer member so that a part of the developed layer becomes attached to the intermediate transfer member, thereby preventing an accurate detection of the developed amount of toner.
Transfer characteristics are greatly influenced by the attaching force between the toner and various members, and are more sensitive than the development characteristics to the detection of a change in powder characteristics due to degradation. It is necessary, as mentioned above, to detect the developed amount and the transferred amount or the residual toner amount when detecting the transfer characteristics. This results in an increase in cost to provide plural detecting elements, or limitations in the type of detection system in which such detection can be made although there may be no increase in the number of detecting elements.